This invention relates to rechargeable electrochemical cells of the lithium-ion or lithium-alloy type.
The use of non-aqueous electrolytes has allowed the development of high voltage lithium-based electrochemical cells for energy storage. Such cells are further characterised in that their electrodes may be intercalation compounds. The positive electrode structures may be based on transition metal oxides operating at a potential close to 4V vs. Li/Li+. Negative electrode structures of carbons and graphites may be applied, which reversibly intercalate lithium at a potential close to the potential of metallic lithium. Such cells are referred to as lithium-ion cells, as the active lithium is always in its ionic form. Alternatively, alloy negative electrode structures like Lixe2x80x94Al and Lixe2x80x94Sn may be used. Such cells will be referred to as lithium-alloy cells. All of the above configurations provide voltages close to 4V.
For the cells referred to above one of the limiting factors for their energy density has been a low initial capacity retention. Upon operation, a capacity loss during initial charging of the cells is observed, as is a fading capacity upon extended cycling or storage, which in combination define the initial capacity retention.
The capacity reduction phenomena are ascribed to the instability of the electrolyte towards the electrodes. Instability towards the negative electrode leads to gassing and formation of a passivating film, whereas instability against the positive electrode leads to corrosion of the electrode structure. Both phenomena involve electrolyte decompositon and result in loss of active lithium and a fading capacity of the cell.
In lithium-ion cells, the losses from the anode reactions dominate the losses at the cathode. The magnitude of the losses merely depends on the type of carbon(s)/graphite(s), the electrolyte and their combination. Using carbon-based anodes, active lithium corresponding to 30-50% of the amount of active lithium in the cell may be lost during the first charge-discharge cycles of the cell, i.e. during the initial charging and the young life of the cell. The use of graphites permits somewhat lower losses in the range 5-30%, however, with poorer long term capacity retention.
In the lithium-ion cell active lithium is provided solely via the cathode. Although prelithiation of carbon/graphite anode structures has been investigated, traditionally lithium-free carbon/graphite structures are applied. Compared to cells based on pure metallic lithium, the loss of active material is rather detrimental. Whereas metallic lithium can be added at 3,800 mAh/g, the specific capacities of the cathode materials are significantly lower.
Currently, LiMn2O4 is one of the active cathode materials used in lithium-ion cells. The active lithium capacity thereof depends to some extent on the preparation method, but is generally of the order of 122 mAh/g.
Therefore, simply providing additional LiMn2O4 to compensate for any loss of active material is somewhat inefficient and may reduce the lithium-ion cell capacity and energy density significantly.
Losses occur in the lithium-alloys cells, too. In the alloy cells with which the present invention is concerned, the lithium alloys are formed in-situ, as this obviates the need for the difficult handling of low potential lithium compounds, e.g. under inert conditions. In such cells active lithium is provided solely via the cathode.
In one type of alloy cell the base material is provided as an oxide. In the case of tin, the reaction scheme is:
4 Li+SnO2xe2x86x922Li2O+Sn
4.4 Li+Snxe2x86x92Li4.4Sn
This scheme clearly shows the irreversible loss of lithium in terms of lithium oxide, in this case being in the range of 48% of the total amount of active lithium.
In another type of lithium-alloy cell lithium is simply alloyed into the base metal, such as aluminium or silicon, which is applied directly in the cell. In the case of aluminium, the reaction scheme is:
xLi+Alxe2x86x92LixAl
In such case a loss is observed as the diffusion of lithium in the xcex1-phase of the lithium-aluminium alloy is so slow that lithium therefrom is practically not released during discharge of the cell. Further, the above instability phenomena still exist and cause additional loss of active lithium.
Therefore, there is a need for an efficient concept for providing additional active lithium to compensate for capacity losses in lithium ion cells as well as in lithium-alloy cells. Such active lithium is provided entirely via the cathode.
A number of patents describes approaches to compensate for the loss of active lithium:
U.S. Pat. Nos. 5,429,890 and 5,561,007, both to Valence Technology, suggest the use of LiMO2 additives (""890: Mxe2x95x90Ni,Co and mixtures thereof, ""007: Liy-xcex1-MnO2) to a LiMn2O4 based cathode. As the additives mainly display rechargeable capacity, these patents are merely aiming at closing the voltage gap between the 3 V and the 4 V plateaus of the Li/LiMn2O4 system.
U.S. Pat. No. 5,370,710 to Sony describes a different approach to alleviating the capacity loss, in particular doping of a LiMn2O4 cathode material with an additional amount of lithium to obtain a compound Li1+xMn2O4 compound either by chemical or electrochemical means. A specific chemical doping method is described in U.S. Pat. No. 5,266,299 to Bell Communication Research, which involves doping of LiMn2O4 or xcex-MnO2 with LiI.
U.S. Pat. Nos. 4,507,371 and 5,240,794, both to Technology Finance Cooperation, describe lithium manganese oxides with excess lithium compared to LiMn2O4. ""371 describes cathode structures of Li1+xMn2O4, x greater than 0, whereas ""794 describes a range of compositions within the compositional area defined by the corner compositions Li14Mn5O12, Li2Mn3O4, LiMn3O4 and Li4Mn5O12, including Li1+xMn2O4 where xxe2x89xa70.25.
Although a number of approaches exists for the introduction of additional active lithium into rechargeable lithium cells, there is still a need for additives to cathodes of such cells, which provide high capacity, safe and simple processing and which are low cost compounds.
A number of patents describe the use of alkali metal transition metal oxide cathode materials.
U.S. Pat. No. 3,970,473 to General Electric Company discloses a solid state electrochemical cell, the cathode comprising a non-stoichiometric lithium compound of the composition LixMnyOz, 0 less than x less than 1 and 0 less than yxe2x89xa63 and z has a value to obtain electrical neutrality. Although such compositions include LiMnO2 structures, the patents does not suggest the use of such compounds as an additive to LiMn2O4-cathode structures.
U.S. Pat. No. 4,302,518 by Goodenough and Mizuchima describes an AxMyO2 structure (A: Li, Na, K, M: transitions metal, x less than 1, y≈1) having the layers of xcex1-NaCrO2, which is monoclinic. The patent, however, does not disclose on the use of such compounds as an additive to LiMn2O4-cathode structures.
U.S. Pat. No. 4,668,595 to Asahi describes a secondary battery with a negative electrode of a carbonaceous material and a positive electrode of layered composite oxide of the formula AxMyNzO2, where A is an alkali metal, M is a transition metal, N is selected from the group of Al, In, and Sn, and 0.05xe2x89xa6xxe2x89xa61.10, 0.85xe2x89xa6yxe2x89xa61.00 and 0.001xe2x89xa6zxe2x89xa60.10, respectively. The patent, however, does not suggest composite cathode structures.
U.S. Pat. No. 5,316,875 to Matsushita discloses a process for the lithiation of LiMn2O4, LiMnO2, LiCoO2, LiNiO2 LiFeO2 or xcex3-V2O5 by exposure to butyllithium, phenyllithium or naphtyllithium. The patent, however, does not suggest use of the cathode active materials as additives to LiMn2O4-cathode structures.
U.S. Pat. No. 5,352,548 to Sanyo describes the use of cathode materials selected from V2O5, TiS2, MoS2, LiCoO2, LiMnO2, LiNiO2, LiCrO2, LiMn2O4 and LiFeO2 in a secondary cell with a vinylene carbonate containing electrolyte. The patent does not describe the use of these cathode materials in composite cathode structures.
U.S. Pat. No. 5,358,805 also to Sanyo describes the use of the materials FeS2, MoS2, TiS2, LiNiO2, LiMn2O3, LiFeO2, LiCoO2 and MnO2 as cathode in a secondary battery with a BC3N-anode. The concept of composite cathodes is not described in this patent.
U.S. Pat. No. 5,478,672 to Sharp describes a secondary cell based on a lithium manganese oxide cathode, which is characterised by having an X-ray diffraction pattern (CuK-xcex1) which shows at least three peaks in the ranges 15.2-15.6xc2x0, 18.6-18.8xc2x0 and 24.5-25.1xc2x0, the lithium manganese oxide typically having a chemical composition LixMnOy, 0.8 less than x less than 1.2 and 1.9 less than y less than 2.2. Although the X-ray diffraction patterns of the above lithium manganese oxide and of the lithium manganese oxide used according to the present invention have at least one common peak, the patent does not describe the use of such compounds in a combined cathode application with LiMn2O4.
U.S. Pat. No. 5,506,078 to National Research Council of Canada describes a method of forming a spinel-related xcex-Li2xe2x88x92xMn2O4 by electrochemical deintercalation of lithium from an orthorhombic LiMnO2 of space group Pmnm and unit cell a=4.572 xc3x85, b=5.757 xc3x85 and c=2.805 xc3x85. Although the unit cell dimensions of the patent is in fair accordance with those of the orthorhombic LiMnO2 used according to the present invention, the patent does not disclose composite cathodes of LiMnO2 and LiMn2O4.
U.S. Pat. No. 5,531,920 to Motorola describes a method for the synthesis of AMO2 compounds, A being selected from the group of Li, Na, K and their combinations and M being selected from the group of Ti, V, Mn, Cr, Fe, Ni, Co and their combinations. The patent discloses a method in which M(OH)2 is reacted with oxidizing compounds selected from the group of Li2O2, LiNO3, LiClO4, Na2O, K2O2 and their combinations. The patent, however, does not disclose the use of such compounds as an additive to LiMn2O4-cathode structures.
U.S. Pat. No. 5,558,961 to The Regents of the University of California, describes an orthorhombic alkali metal manganese oxide secondary cell based on a cathode active material MxZyMn1xe2x88x92yO2, where M is an alkali metal, Z is a metal capable of substituting for manganese such as iron, cobalt or titanium, 0.2xe2x89xa6xxe2x89xa60.75, and 0xe2x89xa6yxe2x89xa60.6, which is initially formed as Na0.44ZyMn1xe2x88x92yO2. Although the patent discloses orthorhombic LiMnO2and its use in electrochemal cells, it does not describe the use of such material as an additive to LiMn2O4-cathode structures. The patent further describes cathodes of lithiated orthorhombic sodium manganese oxide, however, the manganese of the lithium/sodium manganese oxide compounds is in an oxidation state in the range +3 to +4, i.e. higher than the oxidation state of +2 used according to the present invention . Further, the patent does not give any teaching on the use of such material as an additive to LiMn2O4-cathode structures.
U.S. Pat. No. 5,561,006 to SAFT describes a rechargeable cell with a cathode including at least one orthorhombic substance which is a yellow-green single phase oxide of lithium and manganese with lattice parameters a=0.459xc2x10.004 nm, b=0.577xc2x10.004 nm and c=0.281xc2x10.003 nm and a molar ratio of Li and Mn in the range 0.85-1.10. Although the lattice parameters of the black orthorhombic LiMnO2 used according to the present invention are within the range given by SAFT, the patent does not describe the use of the yellow-green oxide as an additive to LiMn2O4-cathode structures.
The present invention provides rechargeable lithium cells wherein the extra cathode material needed to alleviate the consequences of the capacity loss referred to above is one or more components which have higher specific capacities than that of the rechargeable cathode material.
The substituted capacity may be non-rechargeable, since there is no absolute need according to the invention, for the extra cathode material to contribute to the rechargeable capacity of the cathode. The invention is therefore aiming at lithium-ion cells and lithium-alloy cells, which comprise additives, which are able to deliver a substantially higher capacity in the first charge than the rechargeable cathode material itself, but which do not necessarily contribute to the rechargeable capacity upon further cycling of the electrochemical cell. On the other hand, the extra cathode material may contribute to the rechargeable capacity of the cathode and any such additional rechargeable capacity may be beneficial for the performance of the cell.
More specifically the present invention provides electrochemical cells of the lithium-ion and lithium-alloy type, which as additive to their cathode structures comprise one or more compounds selected from a series of alkali metal transition metal oxides which have high first charge capacities.
The present invention provides a rechargeable electrochemical cell comprising a negative electrode, an electrolyte and a positive electrode, characterised in that the positive electrode structure thereof comprises (a) one or more materials selected from the group consisting of LiMn2O4, LiCoO2, LiNiO2, LiNixCo1xe2x88x92xO2 where 0 less than x less than 1, preferably LiMn2O4 and (b) one or more materials selected from the group consisting of orthorhombic LiMnO2, monoclinic LiMnO2 (m-LiMnO2), hexagonal LiFeO2 (h-LiFeO2), xcex1-NaMnO2, xcex2-NaMnO2, xcex1-NaFeO2, and lithium/sodium compounds of the formula LixNayMy(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, and where M(II) is a transition metal in oxidation state +2, selected from the group of consisting of Mn, Co, Ni and Fe, preferably LixNayMn(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa7o and x+yxe2x89xa62, more preferably LiNa0.6MnO1.8, the material(s) of group (a) being present in the electrode structure in an amount corresponding to 20-98% by weight of the complete electrode structure, and the material(s) of group (b) being present in the electrode structure in an amount corresponding to 1-79% by weight of complete electrode structure, with the proviso that in the case of the material(s) of group (b) including any of xcex1-NaMnO2, xcex2-NaMnO2, xcex1-NaFeO2 and LixNayM(II)O1+1/2(x+y), the amount of active sodium originally present in the positive electrode should be lower than the amount of lithium originally present in the electrolyte phase, and with the further proviso, that any material of group (b) in the positive electrode structure should display a higher first charge specific capacity than any material of group (a) in the positive electrode structure.
According to the present invention, an electrochemical cell is provided, containing a cathode structure comprising LiMn2O4 and a material selected from a series of alkali metal transition metal oxides, a non-aqueous electrolyte comprising one or more lithium salts and an anode which comprises an electrochemically active carbon structure selected from the group of graphite, coke and carbon blacks or an alloy based on a metal, in particular aluminium and silicon, or based on a metal oxide, in particular tin oxides.
The alkali metal transition metal oxides used according to the present invention are orthorhombic LiMnO2 (o-LiMnO2), monoclinic LiMnO2 (m-LiMnO2), hexagonal LiFeO2 (h-LiFeO2), xcex1-NaMnO2, xcex2-NaMnO2, xcex1-NaFeO2 and mixed lithium/sodium compounds of the formula LixNayM(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, and where M(II) is a transition metal in its oxidation state +2, selected from the group of Mn, Co, Ni and Fe.
Although other transition metal oxides exist, the above compounds are characterised by having high first charge capacities in practical electrochemical cells, i.e. their practical capacities are close to their theoretical capacities. Further, the extraction of lithium proceeds without deterioration of the rechargeable compounds of the cathode, which are stable against the delithiated additive.
In a preferred embodiment of the invention the additive cathode material is monoclinic LiMnO2.
In another preferred embodiment of the invention the additive cathode material is orthorhombic LiMnO2. Such orthorhombic LiMnO2 may be characterised by one or more of the following features:
(1) peaks of full width at half maximum of less than 0.2xc2x0 at 2xcex8-values of 25.0xc2x0, 39.4xc2x0 and 45.2xc2x0 upon XRD analysis using CuKxcex1;
(2) having been prepared at a temperature higher than 600xc2x0 C. (High Temperature-o-LiMnO2, HT-o-LiMnO2); and
(3) having a mean particle size in the range 20-40xcexc. Alternatively, such orthorhombic LiMnO2 may be characterised by one or more of the following features:
(4) peaks of full width at half maximum of at least 0.25xc2x0 at 2xcex8-values of 25.0xc2x0, 39.4xc2x0 and 45.2xc2x0 upon XRD analysis using CuKxcex1;
(5) having been prepared at a temperature no higher than 600xc2x0 C. (Low Temperature-o-LiMnO2, LT-o-LiMnO2); and
(6) having a mean particle size in the range 5-15xcexc.
In another preferred embodiment of the invention the additive cathode material is monoclinic xcex1-NaFeO2.
In another preferred embodiment of the invention the additive cathode material is selected from the group consisting of lithium/sodium compounds of the formula LixNayM(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, and where M(II) is a transition metal in oxidation state +2, selected from the group consisting of Mn, Co, Ni and Fe, preferably lithium/sodium compounds of the formula LixNayMn(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, more preferably LiNa0.6MnO1.8.
According to the invention, the lithium containing additives compensate directly for the loss of active lithium in the cell. Surprisingly, other alkali metal compounds, and in particular those sodium compounds used according to the present invention, may provide compensation as well.
In the case of sodium, however, it is additionally required that the amount of sodium in the cathode can be accomodated by the electrolyte. During the first charge of the battery, little or no intercalation of sodium into the anode structure will take place, and during the first discharge of the battery, little or no reintercalation of sodium into the cathode structure should be anticipated. Consequently there should be little or no sodium species in any of the electrodes, so that all or almost all of the sodium should be accomodated in the electrolyte phase. Even in this state, the lithium ion conductivity in the electrolyte phase should be sufficient to allow proper cell operation. Therefore, the original amount of lithium in the electrolyte should be higher than the original amount of sodium in the cathode. In this case the sodium salt of the electrolyte is acting entirely as supporting electrolyte, not taking part in any of the electrode reactions.
The findings of the group of Kanoh (J. Electrochem. Soc. 140 (1993) 3162-66) confirm this observation. This group evaluated the selectivity of LiMn2O4 and xcex-MnO2 for Li+ over Na+ and K+. They concluded that especially the lithium-free structure xcex-MnO2 showed high selectivity for Li+, i.e. that in the presence of Li+, no intercalation of Na+ or K+ will take place.
In a preferred embodiment of the invention the negative electrode consists of a coke or a carbon black. In such an electrochemical cell the cathode composition comprises rechargeable material and additive in amounts corresponding to 20-98% and 1-79% by weight of the complete electrode structure, respectively, preferably 50-98% and 1-49% by weight of the complete electrode structure, respectively, more preferably 60-94% and 5-39% by weight of the complete electrode structure, respectively, even more preferably 60-80% and 10-30% by weight of the complete electrode structure, respectively, with the proviso that in the case of the group of additives including any of xcex1-NaMnO2. xcex2-NaMnO2, xcex1-NaFeO2 and LixNayM(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, and where M(II) is a transition metal in oxidation state +2, selected from the group of Mn, Co, Ni and Fe, the amount of active sodium originally present in the positive electrode should be lower than the amount of lithium originally present in the electrolyte phase.
In another preferred embodiment of the invention the negative electrode consists of a graphite. In such an electrochemical cell the cathode composition comprises rechargeable material and additive in amounts corresponding to 20-98% and 1-79% by weight of the complete electrode structure, respectively, preferably 50-98% and 1-49% by weight of the complete electrode structure, respectively, more preferably 80-98% and 1-19% by weight of the complete electrode structure, respectively, even more preferably 80-89% and 1-10% by weight of the complete electrode structure, respectively, with the proviso that in the case of the group of additives including any of xcex1-NaMnO2, xcex2-NaMnO2, xcex1-NaFeO2 and LixNayM(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, and where M(II) is a transition metal in oxidation state +2, selected from the group of Mn, Co, Ni and Fe, the amount of active sodium originally present in the positive electrode should be lower than the amount of lithium originally present in the electrolyte phase.
In a preferred embodiment of the invention the electrolyte of the electrochemical cell comprises one or more non-aqueous solvents selected from the group of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), xcex3-valerolactone, xcex3-butyrolactone and one or more salts selected from the group of LiCF3So3, LiAsF6, LiBF4, LiPF6 and LiClO4.
In an alternative embodiment of the invention the rechargeable material is selected from the group of LiCoO2, LiNiO2, LiNixCo1xe2x88x92xO2 where 0 less than x less than 1. These materials have capacities in the range 140-160 mAh/g.
In the following tables and examples LiNa0.6MnO1.8 is used as a representative of the group of lithium/sodium compounds of the formula LixNayM(II)O1+1/2(x+y), where xxe2x89xa70, yxe2x89xa70 and x+yxe2x89xa62, and where M(II) is a transition metal in oxidation state +2, selected from the group of Mn, Co, Ni and Fe. The use of LiNa0.6MnO1.8 as a representative should not be considered as any limitation of the scope of the invention.
Table I below summarises the theoretical and actually measured initial capacities of the additives of the present invention. The specific capacities (mAh/g) are measured in half cells with lithium metal negative electrodes at low rate (C/50). They are determined from the amount of charge passed across the cells in the first charge half cycle and the mass of the active material in the cathodes. The initial capacities are obtained upon first charge to 4.3V, 4.5V and 4.7V vs. Li/Li+, respectively.
As can be seen from Table I, the first charge capacities of the cathode additives used according to the present invention are significantly higher than the capacity of LiMn2O4.
According to the invention, the first charge capacities of any of the cathode additives should be higher than the first charge capacities (equal to the rechargeable capacities) of any of the rechargeable cathode materials, with which the additives are used in composite cathodes.
Table II below summarises actually measured rechargeable (reversible) capacities of the additives used according to the present invention. The specific capacities (mAh/g) are measured in half cells with lithium metal negative electrodes at low rate (C/50). They are determined from the amount of charge passed across the cells in the first discharge half cycle and the mass of the active material in the cathodes. The reversible capacities were measured upon cycling in the potential range 3.5-4.3 V vs. Li/Li+.
As can be seen from Table II, the cathode additives of the present invention display small but significant rechargeable capacities.
Table III below summarises the initial (first charge) capacities of composite cathode structures comprising mixtures corresponding to 1:1 combinations (by charge) of the indicated cathode additive and LiMn2O4.
The 1:1 combination by charge used in table III and below in table IV and examples 13-16 illustrate the preferred cathode composition in the case of approximately 50% irreversible loss at the anode. In such a case the reversible and irreversible capacities of the cathode would match the reversible capacity and irreversible loss at the anode. The use of the 1:1 by charge combination should not be considered as any limitation of the scope of the invention.
The capacities were measured in half cells with lithium metal negative electrodes at low rate (C/50). They are determined from the amount of charge passed across the cells in the first half cycle and the total amount of oxide in the cathodes structures, i.e. they are specific initial capacities (mAh/g) for the 1:1 by charge composite of additive and LiMn2O4. The initial capacities are obtained upon first charge to 4.5V and 4.7V (vs. Li/Li+), respectively.
As can be seen from Table III, the first charge capacities of the composite cathode structures of additives of the present invention and LiMn2O4 are significantly higher than the capacity of pure LiMn2O4.
Table IV below summarises actually measured rechargeable (reversible) capacities of composite cathode structures comprising mixtures corresponding to 1:1 combinations (by charge) of the indicated cathode additive and LiMn2O4. The specific capacities (mAh/g) are measured in half cells with lithium metal negative electrodes at low rate (C/50). They are determined from the amount of charge passed across the cells in the first discharge half cycle and the mass of the active material in the cathodes, i.e. they are specific reversible capacities (mAh/g) for the 1:1 by charge composite of additive and LiMn2O4. The reversible capacities were measured upon cycling in the potential range 3.5-4.3 V (vs. Li/Li+).
As can be seen from Table IV, the reversible capacities of the composite cathode structures are in accordance with the sum of reversible capacities of the individual components, weighted by their relative abondance.